Let's say scientists created a pill, that would satisfy our daily nutritional needs. It would have all vitamins needed, all calories, minerals - everything that a human body needs in order to function properly. The side effects are unknown.

How much is this pill in the "fiction" section? Is it really possible to create such pill?

How would this pill change our diets? I believe the price would have a great impact, so let's discuss three possible prices: 1, 100, 10000 american dollars. But would it replace natural meals entirely?

Assuming that this pill is used by 25% of Earth's population for a long period of time (100 years). Would they look, feel, behave different than the rest of humans? What about 100% of Earth's population?

EDIT: My thoughts:

This pill would cause obesity for some people - using it wouldn't make you feel "full" so you could just eat the same amount of food that you ate previously.

Many people would be affraid of side effects which are currently unknown

This would solve hunger in third world countries, IF the prices would be right. This might cause economic growth.

The pill could improve or even save million of lives, so I'm guessing the price would be high (maybe thousands or dollars), so only the wealthy ones would afford it.
But the wealthy ones have enough money to buy food on the regular basis so they could use it only if they were to travel to space or somewhere where food would be not so easily available.

$\begingroup$Note that even in 24th century Star Trek, some people make a point of making "real food" rather than "replicated stuff". In our world of today, quite a few people (for varying reasons) seek to live a simpler life using primarily or exclusively the technologies of yestercentury. Assuming such a pill can be made, I think it's safe to say that something similar would happen in your world: even if it is widely adopted, there will be some fraction of people who will shun it, for any number of reasons.$\endgroup$
– a CVnAug 7 '15 at 9:20

5

$\begingroup$This seemed to be a common idea of what the future would be like, in the 1960s. It fails to note that people actually enjoy eating.$\endgroup$
– David RicherbyAug 7 '15 at 17:18

$\begingroup$Agree with @jamesqf ... Fiber is the down fall of any kind of purely liquid diet. Your body isn't going to be happy shutting down the solids processing heh.$\endgroup$
– KaitharAug 7 '15 at 19:33

$\begingroup$@Kaithar - I'm not so sure. Studies of people adrift on rafts without food show that defecation ceases but starts up again naturally when food is eaten once more. (remembered facts from reading years ago) I think that the body would cope with pure nutrients.$\endgroup$
– chasly from UKSep 2 '15 at 12:59

No

The problem is that very nutrient dense foods suck water out of your body through osmosis. This leaves the consumer dehydrated and they still aren't able to absorb the nutrients.

If you perform the math on the numbers provided in the answer linked above and for a 2000 kcal diet (average for a male), the densest nutrient concentrations a person could easily tolerate would require them to consume about 6-8 $Ensure^{TM}$ per day. The total mass of this would be $7 \times 240 g $ ~ $1.7 kg$. To this you'd need to include an extra supply of water since the quantity provided in $Ensure^{TM}$ would be insufficient to sustain a person.

I realize this number seems high, but realize that this 1.7 kg (3.7 lbs) of food includes 1.3 liters (2.8 lbs) of your daily water requirements.

Yes

Instead of consuming your "food pill" directly, if you instead dropped it into a container of water and drank the results, that would probably work if you got the quantities right.

In this case, your "food pill" for a day's rations would mass about 0.4 kg (0.9 lb) and would need to be dissolved in the quantity of water identified above.

Other Stuff

Preferences

As a person with digestive issues, I've been forced to use $Ensure^{TM}$ to supplement my diet. I never drank it because I liked it. This stuff tastes OK but it isn't something most people would choose over regular food.

Long-term Use

Perhaps more important, we still do not know all the nutrients the human body needs to stay healthy. We know the macronutrients (fat, carbohydrates, protein, and water); we know the micronutrients (vitamins & minerals) but we do not know the trace nutrients that are beneficial.

The trace nutrients are those chemicals are the ones scientists look for in wine, coffee, chocolate, etc. Scientists are constantly changing their minds about whether this food or that are good or bad for you.

A diet of only these concentrates will keep you going but might lead in the long-term to a variety of difficult to diagnose issues. We just don't know enough about it right now.

Costs

Right now, these concentrated foods are substantially more expensive than the food that they'd replace (perhaps $10 / day). If you wish to solve world hunger it does have some advantages though:

Light weight - easy to transport

Shelf stable - no spoilage

Solving Obesity

As our knowledge of obesity increases, this answer may become dated but current research implicates many factors of which layman are unaware.

Of these, the "food pills" might only solve #1 or possibly #5 if the "food pills" were laced with gut biota known to be a good & healthy mix.

I suppose it's possible that a sophisticated society could generate "food pills" based upon a doctor's prescription in which case #3, #4, & #6 might also be addressed by taking your and only your "food pills".

Hunger

as a motivational state.

The hunger mechanism in people is fairly complicated, can be triggered or suppressed by several different factors, and has different processes over short and long terms.

A food pill can be designed in a way that triggers all the right GI tract and blood monitoring triggers as long as the "pill" contains "bulk" for the GI tract to work on. Fiber would fill this need quite well. Furthermore, current diet nutritional shakes already include fiber in their formulations for just this reason.

Eating as a sensory experience

I think one area in which this idea fails would be when "eating as a sensory experience" When humans really indulge in the pleasures of eating, we engage more senses than just taste. We enjoy different textured food (touch), foods with pleasing aromas (smell), colorful arrangement (sight), and even sometimes foods that make sounds when eaten (sound).

Food pills might be able to accommodate taste and to a lesser extent smell but would leave out the other senses. For these reasons (and a few others), I don't think food pills would ever replace "real food" on the luxury market.

$\begingroup$The Factors "Causing" obesity you listed are actually only contributing factors. No genetic predisposition can make you gain wait if you are not exceeding your calorie needs, they can only be more or less friendly in today's calorie rich diets. Nothing can make calories out of nothing. Diet alone CAN prevent obesity in all cases if tailored to a person's individual needs. Now, there a many other serious health concerns that those other issues can cause that diet CAN'T fix, but obesity is not one of them.$\endgroup$
– wedstromAug 7 '15 at 17:42

$\begingroup$@westrom, actually research indicates this isn't true. That when subjects are fed the same diet some will get fat and others will stay a health weight. In my own personal case, I need to consume about 3000 kcal / day to maintain weight because my body has difficulty absorbing the nutrients. There are many factors and they interact in ways we can't predict. We are discovering that "blame the fat person" is very often wrong. Perhaps in 20 years we'll know enough to assign blame where it really belongs.$\endgroup$
– Jim2BAug 7 '15 at 21:11

$\begingroup$@kaithar, due to damage to my gut, I've spent many years (5-10) with extremely low fiber diets with little directly quantifiable ill effects. These very low fiber diets increase your odds of various ailments but in some cases those risks are a better choice than the alternatives (intestinal blockage is extremely painful, potentially life threaten, and can require emergency major surgery to correct).$\endgroup$
– Jim2BAug 7 '15 at 21:13

1

$\begingroup$As for Ensure--I've had to do worse: Vivonex. It's basically pure nutrition. I don't think any adult would need less than a pound a day of it. That's an awful lot of pills! You could get somewhat smaller by getting most of your calories from fat (Vivonex has a little fat as medically acceptable) but you're still going to be near a pound for an average adult.$\endgroup$
– Loren PechtelAug 10 '15 at 1:31

While the vitamins would probably be relatively cheap and easy to supply, calories would be another matter.

The most calorie-dense foodstuff is pure lard, at about 892 calories per 100 grams; for reference, a stick of butter usually weights around 100 grams.

An average adult human needs between 2000 and 2400 calories every day, so someone looking for the bare minimum calories would need to eat bout 224 grams of pure fat. This would obviously be too much for one pill to handle.

For the sake of creating a fictional world, you could possibly get around this with the invention of a new kind of synthetic organic molecule which packs about 2000 calories into a single gram, with a second gram used for traditional multivitamins.

This would solve the energy issues, but it would still leave people with too little actual matter for their bodies to work with; they wouldn't be able to do things like develop muscle or hydrate themselves. The best the pill could do would be stave off starvation.

One pill a day to satisfy a person's nutritional needs is a very old trope in SF. The problem is that physics, chemistry and human physiology (and maybe psychology) simply don't support it. Having a human survive on just a single pill every day without any supplemental energy sources is complete and utter fiction.

Limited Nutrient Compressibility

DIY Soylent is an excellent place to experiment with different nutritional profiles. For example, the recommended nutritional intake for an adult male for a vitamin C alone is 2 grams. The density of Vitamin C is 1.69 g/cm³. Even without doing any math to find out exactly how big the pill is, it's becoming larger than many people will want to swallow, and this is just one nutrient! For an idea of scale, the below photo is 1 cubic centimeter cubes in a child's hand:

Granted, many other nutrients take up far less space, biotin in particular is needed in vanishingly small amounts. These small nutrients are just statistical noise compared with the larger nutrients like calcium carbonate at2.71 g/cm³ and an adult male needs 1 gram. Females need more than that. Calcium is a sizable pill even without binders to keep it all together.

Calorie Requirements

Let's start with the standard 2000 calorie diet in the form of protein, fats and carbs. The human body needs a certain amount of protein every day or it will start consuming its own muscles to get it. While human muscles and organs can survive on ketone bodies (the energy transport mechanism for energy derived from fat) the brain requires sweet, sweet carbs. (From personal experience, trying to fuel the brain on just ketones won't work. Just don't do it. Sugar cravings like you wouldn't believe!) Thus, we have minimal intake requirements for protein and carbs; all the other energy requirements can be made up with fats/oils/lipids.

For quick reference, carbs are 4 calories (nutritional calories, not physics calories) per gram, protein is 4 calories per gram, alcohol is 7 calories per gram and fats are 9 calories per gram.

Without addressing macronutrient ratios, let's just assume that all calories will come from fats thus giving a lower bound on the required mass to fuel a human body for a day. 2000 calories / 9 calories/gram = 222.2 grams of fats. Using canola oil (at 0.92 g/ml) for fuel gives us 241 cubic centimeters. For comparison, a Rubik's Cube is a mere 185cm^3. No one swallows Rubik's cubes.

Mush, no matter how sweet is not tasty

As this scene from the Matrix so perfectly describes, humans don't like eating tasteless, monotonous slop. Sure they will if they have to, to survive but they won't like it. From an evolutionary biology perspective, it's not hard to make the jump that an early human that ate the same thing every day, all day soon developed nutritional deficiencies and could not compete. An instinct for varied foods and varied textures helps cover nutritional deficiencies.

But what if you could do it all in a pill...

Okay, okay, what if you could make a pill that filled a person's nutrient and caloric needs for one day?

A great deal of nutritional and inflammation based diseases would go away.

The obesity epidemic in the US would vanish.

The populations body fat percentage would settle at levels particular to that person's genetic. Ectomorphs would stay thin. Endomorphs would stay heavy. In both cases, they would be more easily able to accept their body shape because they know they're giving their bodies what they need/want.

Outside of issues like how many calories a pill could pack, the human body has evolved to process natural food. We need a certain amount of fibre in order to push things through the intestines, for example.

The other issue is that different humans have different needs. You can eat things off a plate or ignore them, and order individual meals to meet dietary, social, religions or other requirements. I'm not sure many people would believe your pill is Kosher, Gluten free or an appropriate snack food for watching the game. Taking a prospective mate to dinner would be a totally different thing (think of all the rituals involved around taking a date to dinner).

Pill food might be appropriate for emergency situations (for example, a box of emergency rations in a survival kit would be much more portable if they could be replaced by pills), or perhaps for extreme adventurers (hiking with a pack of pills across the Amazon rain forest), but even then, the social aspects of sitting around the camp at night and sharing a meal wold be lost.

So the other answers have made it pretty clear that you can't pack a full day's nutrients into a single pill, even if it's a large "horse pill".

But what if, instead of digestible nutrients, this pill contains an array of chemical- or nuclear-fueled nanomachines and a stock of raw elements? What if these nanomachines could use these elements together with some amount of ingested water to build nutrients and then exhaust them into the stomach? Then, later, when the machines run out of fuel, they make their way naturally down the digestive tract to be excreted.

As for the effects of such a technology:

Obviously the users of such a pill would stop defecating for the most part. Reportedly this already happens with soldiers who subsist entirely on military rations. That alone would save humanity a lot of time and water, which would contribute positively to the economy.

If the pill was very cheap, it might result in a population explosion. If it was very expensive, most likely no one would really use it, and it would remain a laboratory curiosity except in certain contexts where space and weight is at a premium, for example, space travel.

It's not certain what problems prolonged use might entail. Obesity rates might drop, since it's a (very) controlled portion and it kind of removes the fun from mealtimes. Even if it's pretty safe, the medical profession might recommend "real" food every once in a while to flush out the system.

It is complete fiction with today and near-term technology. I think most current answers cover that aspect.

If you were looking far into the future though, it is theoretically possibly that a bionic upgrade could allow you to just shove your daily energy intake, as a pill, into a hatch which would slowly push the nutrients into your body throughout the day. Depending on just how advanced we can get with stuff like that, maybe the bionic upgrade completely optimizes and replaces our current digestive system for our processed bricks of material.

How much is this pill in the "fiction" section?

Very much so, unless you plan on taking a very large pill every 4 minutes. (This number comes from @Greens answer, which suggests that 241cm^3 sized pills would be needed for a day's worth of Calories. Subtracting 8 hours for sleeping, you could take 1cm^3 pill every 4 minutes to get the correct amount. - but then you have to add in all the other stuff your body needs as well, likely making that pill much larger

How would this pill change our diets?

We wouldn't have to worry about our diets! That and the pill being slightly more compact than normal food are really the only positive things about it.

But would it replace natural meals entirely?

Unlikely, people like to eat and try new tastes. It makes us feel good to eat. Doing otherwise just isn't "natural", there would have to be a huge reason not to do it.

Assuming that this pill is used by 25% of Earth's population for a long period of time (100 years). Would they look, feel, behave different than the rest of humans? What about 100% of Earth's population?

If you go with the bionic thing, then people would look very different due to that - but people's looks wouldn't drastically change because of what form they are getting their food in, especially not in only 100 years.

No one is thinking long term here. It's my understanding that one of the most important differences between us and other species is the cooking of foods with fire. Leading to smaller intestine and bigger brains. If a pill or several pills taken throughout the day could replace our current food situation wouldn't it seem to imply even less intestinal need and even bigger brains?????

When I was in my twenties I experimented for several weeks by surviving only on milk mixed with a well-known British energy drink (Lucozade) supplemented with vitamin pills. At the time I used to cycle to work ending with a hill.

At the end of the trial I felt fitter and was slimmer than when I started. I had no adverse symptoms at all. (see disclaimer)

The biggest effect of all came from joining colleagues at mealtimes. After only four days I lost any cravings for what they were eating but it was just so boring. The whole business of eating is important socially or alone and was the reason I stopped. I had a huge desire to chew and swallow and even to use eating utensils.

Disclaimer

I am not in any way medically qualified. I was young and in good
health at the time and I cannot say what effects, good or bad, such a
diet would have on anyone else.